In order to control the printing process, and for the quality analysis of printed products, it is known to print, alongside the actual subject, measurement fields, which can be scanned by means of measuring instruments. For this purpose measurement fields are preferably combined, at least section by section, to form measurement strips, that is to say the individual measurement fields are lined up to form a strip. For the purpose of monitoring offset printed products, use is mostly made of densitometers, in which the reflectances of the measurement fields are registered by filter or measurement channels configured to be complementary to the printing inks, and are converted to an ink density value by means of taking logarithms. The particular advantage of ink density measurement is the fact that the density value has a simple relationship with the ink layer thickness. The densitometers used today have a very high resolution rate over time. That is to say, it is possible for a large number of measured values to be obtained on a measurement field of given size.
For the analysis of measurement fields lined up with one another to form color measurement strips, measuring systems move in one or two directions, usually by way of a measuring head (densitometer, spectrophotometer) that moves in the X-Y plane. For this purpose, the measuring head is movably suspended on a bridge that spans the printed product. Part of the bridge may move with respect to the substrate supporting the printed product. The measuring head can be moved relative to the bridge, and the bridge can be moved relative to the substrate. Associated actuating motors provide the drive source.
By means of a measuring system of this type, it is possible for several measurement strips distributed over a printed sheet or individual measurement fields to be scanned section by section. The measurement sequence is programmed in advance and then executing under automatic control during the actual monitoring of the printed products.
The many advantages of densitometry for printing control are countered by the fact that the ink density values cannot be converted to calorimetric values or compared with calorimetric values without making certain assumptions. The reason for this is that colorimetry is based on filters that are adapted from the human color sensation (i.e., X-, Y-, Z- standard spectral value curves). In order to measure calorimetric values, that is to say color loci, it is necessary to expand a densitometer (with three or four channels) to include a color measuring system. The coloring measuring system may be designed as a tristimulus measuring instrument or spectral measuring system. It is also possible to design the measuring system as a spectral measuring head or spectrophotometer, wherein the weighting of the received reflectances are carried out purely by computation--i.e., the registered spectra are weighted by predefined filtering functions.
However, the integration or computation times for weighting the reflected spectra detected by such spectrophotometers are several times greater than those of known densitometers. Thus, fewer spectra data are received per unit time with respect to density data, and there are also fewer data available for further analytical purposes. This disadvantage is serious if such a measuring system of spectrophotometric design is used in a so-called scanning measuring system, in which the measuring head is moved relative to the measurement fields of the measuring strip.
In order to minimize the space needed on the printed material by a measurement strip, the measurement fields are provided with the smallest possible dimensions. For a typical traveling speed of the measuring head relative to the printed material and the measurement field thereon, there is only a short time available for measurement. In the case of the long integration or computation times of a measuring system with a spectrophototmeter and the resulting low sampling rate, there are only a few samplings of the spectra data available per measurement field.
A further disadvantage of a measuring head that employs a spectrophototmeter are the polarization filters needed by the densitometer to eliminate gloss effects induced by fresh ink, which cannot be used by the spectrophotometer for ascertaining color loci. A spectrophotometric measuring instrument which, from the reflectance values, permits both ink density and calorimetric measured values to be obtained by calculation, is already known, for example as descried in U.S. Pat. No. 5,182,721 (EP 0 228 347).
A combined measuring head, comprising a densitometer for obtaining ink density data and a tristimulus measuring head for measuring calorimetric values, is described in U.S. Pat. No. 5,141,323 (DE 3 830 731 A1). This patent also discloses fitting such a combined measuring head to a measuring system that can be moved relative to the printed product. The disadvantage of this approach, however, is that the tristimulus measuring heads are miniaturized, which results in their not satisfying the requirements for accurate measurement. Furthermore, in the case of a tristimulus measuring head, different illumination conditions and assessment functions can be realized only with a relatively high outlay, which also is contrary to miniaturization.
German Patent No. DE 195 30 185 C2 discloses a spectral measuring head alongside an ink density measuring head of integral construction. In this construction, separate optical channels are used for obtaining spectral reflectances and ink density measured values. The disadvantage to this approach, however, is that, as already indicated above, the integration or computation times are very different for obtaining the values of the ink density the spectral reflectance.
Also, in order to obtain calorimetric measured values by means of the spectral measuring system, the measuring head must be adjusted relatively precisely to the measurement field to be measured, which makes an automated method more difficult. Precise adjustment of the measuring head is necessary in order to correctly measure the measuring fields. Faulty measured values may occur when a measurement is made at a boundary of a measurement field. In this situation, a color is ascertained that corresponds to a mixture of the colors of the measurement field and the printed material or a mixture between the colors of two adjacent measurement fields. These boundary measurements may be detected by evaluating the course of measured values for the ink density obtained shortly beforehand, and thus avoided by calculating the correct measuring position.
The prior art discloses numerous solutions in order, using movable densitometers, to establish both the boundaries of the measurement fields and the location and orientation of a color measurement strips on the printed material. For example, a movable densitometer is known in which the printed sheet with the measurement strip located on it is first placed along a support rail. Then the densitometer carries out a search run perpendicular to the edge of the sheet (i.e., the Y direction), which is transverse to the direction in which the measurement strip extends (i.e., the X direction). This search run in the Y direction determines the distance of the measurement strip from the edge of the printed material. Following the search run, a scanning operation is performed by densitometer by its movement in the X direction (direction of the measurement strip).
It is also known to provide a movable densitometer with a seeking optical unit that comprises a projection device, by means of which two light spots are projected onto the printed material in front of the densitometer in the scanning direction. These light spots having a spacing which is less than the width (X direction) of the measurement fields to be measured. If a densitometer that is equipped with a seeking optical unit moves along the center of the measurement strip to be measured, then the two light spots projected by the optical unit is are both incident on the field to be measured and, thus the reflected light from the two spots is the same. The similarity of the reflected light can be established by means of associated detectors. If the densitometer comes off track, then one of the light spots will move onto an unprinted area of the printed material, whereas the other light spot still reflects the color of the measurement field. In this situation, the different reflectance values determine in which direction the densitometer must be moved in order to correct its position so that it follows along the center of the measurement strip.
Furthermore, during a measurement run of densitometers that operate with high local resolutions, the accumulating measured data are processed in order to establish the color and determine the boundaries or centers of the measurement fields. This determination is carried out by first analyzing the measured values of the ink density obtained from several channels, and then differentiating the values over the course of the measuring head traveling in the scanning direction. By sensing the measurement field boundaries in this way, the center of the measurement field may be precisely determined. Since this technique accumulates a large number of data accumulate for each measurement field, averaging achieves greater reliability of the measured values.